As an apparatus for measuring deformation (strain) of a measured object, a metal foil strain gauge in which a metal resistor (metal foil) is arranged on a thin insulating body has conventionally been well-known. The metal foil strain gauge is adapted to measure changes of an electric resistance value along with deformation of the metal foil in conformity with deformation of the measured object and convert the changes into a strain quantity and has widely been used due to a simple structure, a low price, and high accuracy. The metal foil strain gauge, however, is disadvantageous in that the configuration of the metal foil strain gauge easily causes a measurement error when the temperature of the measured object changes, in that the metal foil strain gauge consumes too high power to be driven at all times, and in that the metal foil strain gauge requires a certain installation area size.
As an apparatus for overcoming these disadvantages of the metal foil strain gauge, a semiconductor strain sensor is developed including a strain detection region (bridge circuit) constituted by impurity diffusion resistors formed on a surface of a semiconductor substrate. The semiconductor strain sensor can even detect ultra-fine strain (that is, has an advantage of being sensitive to strain) since the resistance change rate of the impurity diffusion resistor of the semiconductor strain sensor to strain is tens of times as high as that of the metal resistor of the conventional metal foil strain gauge. Also, with use of a so-called semiconductor process such as photolithography for formation of the impurity diffusion resistor, ultra-fine patterning of the impurity diffusion resistor is available. Thus, size reduction (area reduction) of the entire semiconductor strain sensor and electric power saving can be achieved. Further, ultra-fine patterning of the impurity diffusion resistors enables all of the resistors constituting a Wheatstone bridge circuit to be formed on the same substrate. Thus, fluctuation of electric resistance along with temperature changes of the measured object is canceled out, and this brings about an advantage of a decrease in measurement error (improvement of measurement accuracy).
For example, JP 2007-263781 A (PTL 1) describes a mechanical quantity measuring apparatus including a strain detection unit on a surface of a semiconductor substrate and attached to a measured object to measure strain. In this mechanical quantity measuring apparatus, a semiconductor single-crystal substrate is provided with at least two bridge circuits. One of the two bridge circuits is constituted by n-type diffusion resistors in which a direction (longitudinal direction) of measuring fluctuation of a resistance value while supplying current is parallel to the <100> direction of the semiconductor single-crystal substrate. The other bridge circuit is constituted by p-type diffusion resistors in combination in which a longitudinal direction is parallel to the <110> direction. According to PTL 1, a strain component generated in a measured object in a specific direction can be measured accurately (refer to the abstract).
Also, JP 2012-47608 A (PTL 2) discloses a mechanical quantity measuring apparatus using a bridge circuit formed on a semiconductor substrate. The bridge circuit consists of four bridge resistors Rv1, Rv2, Rh1, and Rh2. Each of the bridge resistors consists of a plurality of diffusion resistors. The plurality of diffusion resistors are arranged on the semiconductor substrate in a matrix form. The bridge resistors Rv1 and Rv2 are the plurality of diffusion resistors arranged in the odd rows of the matrix and selectively connected in series while the bridge resistors Rh1 and Rh2 are the plurality of diffusion resistors arranged in the even rows of the matrix and selectively connected in series. According to PTL 2, it is possible to prevent offset output of the bridge circuit caused by stress generated by temperature changes of a measured object, thermal distribution on the semiconductor substrate, and impurity dosage gradient of the diffusion resistors.
Meanwhile, a fundamental point in mechanical quantity measurement by means of a strain sensor is deformation of the strain sensor in conformity with deformation of a measured object. To detect finer deformation and achieve accurate mechanical quantity measurement, ultra-fine resistance changes of resistors used in the sensor are required to be detected. In this respect, in JP 2013-205403 A (PTL 3), for example, sensors for measuring mechanical quantities are mounted on a plurality of locations of a measured object, and an operational circuit connected to the output of the sensors performs operations, to enable accurate detection.